You can't. It's random unfortunately. Just keep leaving the fight and going back in until you have someone you are good with.
As for the 300% damage healing Scorp. I used Shujinko. But I suggest using your best characther. You might also want to take the easy way out and put your game on Novice for the time being.

...You cannot just select the fighter of your choice. The concept of the Konquest mode in this game is to learn the Mortal Kombat stoyline. Plus, it was ment to be another way of learning how to use the fighters in a fun way.